Saturday, June 7, 2014

Waterjetting 22a - Mining horizontal coal

Over the last few posts I have discussed some of the problems that arise in dealing with the use of waterjets in mining coal, when the material mined has to be collected and transported away from the face where the coal is extracted. I thought I would follow on that thread in a few more posts, ending up, hopefully, where I began back in the process of removing thin layers of material (such as rust) from flat surfaces.

But to get there I am first going to go back to coal mining. One of the problems with adapting what we might call conventional hydraulic mining to coal is that many of the coal seams around the world are relatively flat – it is, after all, the way in which the vegetation that became coal was laid down. Thus the gravity that can be used in a steeply dipping seam as a way of carrying away the coal and the water together, is not initially that helpful.

There are several different ways that have been suggested over the years to solve the problem. Initially these were based on existing mining machines, and methods for mining the coal, but with the teeth of a conventional machine replaced with high-pressure waterjets. One such, as I have written earlier was the MS&T Hydrominer, where the cutting teeth along the edge of a coal plow were replaced with oscillating dual-orifice waterjets to cut a kerf around the coal being mined.

Figure 1. Artist’s impression of the initial Hydrominer, with jets cutting a slot one foot deep ahead of the wedge shape of the plow.

The water used was less than that conventionally used on a mining machine to suppress the dust generated as coal is mined from the solid, and the coal loads onto the armored face conveyor on which it rides down the face.

That particular design was based on an earlier mechanical machine, the Meco-Moore, which I had previously seen working on a longwall in the United Kingdom.

Figure 2. Meco-Moore mining machine set up to mine coal. The cutter jibs cut slots and the coal then collapses onto the transverse conveyor.

However this concept required a considerable investment in the supporting longwall equipment both to hold up the roof and to remove the coal. An alternative approach was to continue to conventional roof-and-pillar mining which is the most popular method of underground coal mining in the United States, but again replacing the cutting teeth with waterjets. The first of these was conceived by IIT Research Institute in Chicago, under Dr. Madan Singh.

Figure 3. A high-pressure waterjet continuous miner.

Unfortunately in this configuration the system did not work well. The jet pressures used were too high, and in consequence the volumes of the jets too low to achieve a deep penetration into the coal.

When the jets were replaced with a combination similar to that of the Hydrominer, and in a device we called RAPIERS, a slightly better performance was achieved, but the demand for innovation had, by that time passed for a spell, even though this particular machine was developed with considerable technical input and financial assistance from the Jet Propulsion Laboratory in Pasadena.

Figure 4. Progression of the RAPIERS machine in room-and-pillar mining.

Both of these machines required that a second set of machines sit behind the excavator and carry away the coal that had been mined, again at significant cost, and they also required machines to support the roof.

There is a different type of machine that is often used at the edge of the productive limit of surface mining. As seams near the surface get deeper so the cost of removing the overlying material becomes too expensive to justify continued mining. At that point companies may bring in an auger which can drill long holes into the coal, and remove the material as with conventional smaller augers that might be used for drilling in dirt (or even drilling holes in wood).

Figure 5. Conventional auger mining (Rosamine )

Because the auger drills a hole to the size of the following scroll, it is relatively easy to carry the coal back out of the horizontal hole, which might exceed 300 ft in depth. But there is a problem with the machine, in that the cutting force to push the auger teeth into the coal at the face of the machine has to be carried through the entire string of augers.

Because of the string of segments this becomes more difficult to control with longer depths, and in addition there is a friction loss due to the continual rubbing of the scrolls against the floor and sides of the hole. Together these act to limit the machine range, since there is little to steer the machine other than the direction of the hole, as it deepens.

Figure 6. Picks on the face of the auger, with early jets mounted in the center of the head to cut a central hole.

If, however, the picks on the face of the auger are largely replaced with waterjet nozzles, particularly at the outer edge of the auger, and with the flow directed there, rather than, as shown in Figure 6, towards the center, then an outer free face – up to a foot deep, can be cut ahead of the cutting head. With larger auger heads the nozzles can be placed across the face, to break the rib of coal, should it start to get too large – especially since the coal needs to be fragmented somewhat to feed down the auger.

Figure 7. Waterjets across the face of an auger (courtesy W.A. Summers)

The reduction in the amount of force that this allows on moving the auger into the coal can be illustrated by example. In developing a version of the machine we built an artificial coal face, made up of coal pieces and cement. It is a little more resistive than conventional coal, however the student, Chris Cannon, had little difficulty pulling the machine into the face with a come-along, even though he only had one uninjured arm at the time of the test.

Figure 9. Chris pulling the 2-ft diameter auger into the artificial coal seam.

By confining the coal and water it was possible to recover both, so that the water could, if needed be recycled.

I’ll continue the thread next post.

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